1. Field of the Invention
The present invention relates to an electrochromic printing medium for use in an electrochromic printing process and a method for making the same.
2. Description of the Prior Art
Electrochromic printing is well known in the art. Types of electrochromic printing materials previously known involve coating a leuco dye halide compound onto a substrate. The substrate is then wetted with water just prior to printing so that the current pulse is passed to the substrate whereby the halide can provide an electro oxidation of the leuco dye into a colored dye.
The methods for electrochromic recording taught in the prior art have many variations. However, basically, one or both sides of a sheet of paper is coated or treated with a coating formulation containing at least one colorless color-forming compound (leuco) compound. Electrical current is then selectively applied to the coated side of the paper by some means, for example, a stylus or a printing head to which a voltage can be applied. The application of the voltage causes an electrochromic reaction, involving the leuco compound to produce a visible image corresponding to the design traced by the stylus or by the printing head.
The following items appear to constitute the most relevant prior art with regard to the instant invention.
J. E. Kassner, "The Development of Oxidation-Sensitive Dyes in Support of IBM's Molecular Matrix Technology", J. Imaging Technology, 12(6), 325 (1986) describes a two-stage process in which bromine is electrolytically generated via the oxidation of a bromide compound, followed by a reaction between the bromine and a leuco dye.
U.S. Pat. Nos. 4,374,001 to Bernier and 4,439,280, 4,443,302, 4,309,255 and 4,478,687 to Gender et al. describe various types of leuco dyes in such aqueous/halide-containing electrochromic coating compositions.
The previous electrochromic mediums, however, suffer from several deficiencies. The mediums provide low image formation speed and poor image stability, which may be partly due to the aqueous type coating compositions which includes a reversible oxidation/reduction reaction of the leuco dye. In an effort to reduce the voltage required to form an image, an aqueous coating is employed which allows for such voltage reduction. However, the aqueous coating causes an undesirable curl and/or cockle in the paper substrate and requires drying of the paper substrate. In addition, problems of anode corrosion also necessitate the use of expensive electrode materials.
Thus, it would be desirable to have an electrochromic medium which can use low voltages to form an image, maintains a stable image, and employs a non-aqueous electrolyte. The present invention provides such a material and a process for making the same.